Low output voltage ripple and a fast response to changes in load conditions are two desirable but conflicting attributes for switch mode voltage regulators. FIG. 1 is a simplified schematic diagram of a step-down switch mode regulator 10 with synchronous rectification. Step-down regulator 10 includes input terminals 12a, 12b across which input voltage V.sub.IN is applied; an input filter 13, field-effect transistors ("FETs") 14 and 16; an inductor 18; a capacitor 20; output terminals 22a and 22b, which provide an output voltage V.sub.OUT where V.sub.OUT &lt;V.sub.IN ; and a feedback control circuit 24 to control the duty cycle of transistors 14 and 16. The duty cycle of transistors 14 and 16 is maintained such that either transistor 14 or transistor 16 is on, but due to inverter 26 both are not on simultaneously. FIG. 2 illustrates the current and voltage waveforms for step-down regulator 10. Control circuit 24 regulates V.sub.OUT by maintaining a duty cycle as illustrated by curves showing the on and off times for FET.sub.14 and FET.sub.16. The current flowing through inductor 18 is illustrated by curve I.sub.18 and the voltage at node 28, which fluctuates from V.sub.IN to circuit common, is illustrated by curve V.sub.28. Thus, regulator 10 produces a voltage V.sub.OUT with a ripple as shown in FIG. 2.
Obtaining a low output ripple typically involves the use of a large filter inductor and/or a large filter capacitor, or the use of multi-stage filters. Although these approaches reduce output ripple, a large filter inductor or multi-state filter reduces the speed with which the regulator can respond to large changes in the output load current. A large filter inductor limits the current slew rate, and each filter stage adds a response time delay affecting the control circuit performance. In addition, a large filter capacitor is expensive and adds substantially to the size and weight of the device.
"Active" filters using linear circuitry can be used in principle to reduce ripple without adversely affecting the response speed to changes in the load current. However, if the original ripple is high, the power loss in an active filter is excessive thereby offsetting the efficiency advantages of switch mode regulation.